Installing a electric heater in my bathroom, the kind that go into the wall cavity between the studs. It requires 240 volts and 16-17 amps. What size wire and breaker are needed?

AWG gauge 12 wire can carry a max amp load of 41 and 10 gauge can carry a max of 55 amps. Both are heavier than you really need but if you already have both use the 12. TK will be hear later and can give you a specific on the breaker. I usually go up to the next breaker larger than the load, 20 amp.

Ballenger, can you cite a reference that lists the ampacities you offer?

#12 wire when used in a builidng in the USA is max 20 amps, and #10 wire is 30 amps.

These ampacities are all per the National Electric Code.

Electric heat is to be considered as a 100 % continuous load. If the heater draws 16 amps, then #12 is fine, if the heater draws 17 amps, then #10 is needed.

Tom, go with TK. He is looking at my amp/gauge chart so I'm not sure of my answer on the gauge of wire. If you don't know how to pull 220 from your panel he will describe what's safe for a home owner to try. He says 10 gauge wire and he's the electrician.

To get 240 volts from panel, you install a 2 pole 30 Amp breaker, You will get 240 volts from there. Every other breaker space, is the opposite phase(240)volts no matter where you install breaker.
Like already been said, 3 #10's(2 hots and ground) 1 more #10 for neutral, if needed.